This invention relates to housings for turbomachines such as, for example, gas turbine engines, and, in particular, although not exclusively to housings for compressors of such machines.
During transient conditions of operation such as on start-up or run-down of such machines the temperature of the working fluid in the machine varies relatively quickly. Those parts of the machine in direct contact with the working fluid are thereby heated or cooled relatively quickly. However, the thermal response of components not in direct contact with the working fluid, is much slower and this can cause problems. For example, in gas turbine engine compressors or turbines having a bladed rotor comprising blades mounted on discs and disposed within a housing, the housing tends to heat up very quickly whereas the more massive rotor discs, which are shielded from the hot gases by the blade platforms and interstage spacers, heat up less quickly. The consequence is that the gap between the tips of the blades and the housing varies because of the differential thermal expansion. This can lead to fouling of the blades on the casing, or, if the gap becomes too large, to loss of efficiency or a surge in the engine.
It is known to construct turbine or compressor housings in the form of two radially spaced casings, and to match the expansion of the inner casing to that of the rotor by providing thermal insulating materials on the inner surface of the inner casing and heat sinks on the inner casing to slow down the thermal response of the casing. Such a compressor casing is described in our British Pat. No. 1,501,916. The casing described in this patent comprises an inner casing made up of annular stator rings carrying stator vanes, and the outer casing comprises cylindrical rings or two half casings joined along longitudinal axes and assembled around the outside of the inner casing and bolted to it.
One of the problems with compressor housings employing inner casing segments which locate in an outer casing is that pressurised air leaks into the space between the inner and outer casing from high pressure regions of the compressor and tends to flow back towards the lower pressure stages of the compressor. This reduces the efficiency of the compressor and has a detrimental effect on the overall thermodynamic cycle of the turbomachine.